It is well known that the stress-strain characteristics of soil are dependent upon the stress paths used to test the soil. For example, if a triaxial test specimen is subjected to compression loading, i.e., increasing vertical stress while holding lateral stress constant, the failure stress will be much higher than if the specimen is subjected to compression unloading, i.e., decreasing lateral stress while holding vertical stress constant. Different results will also be obtained by subjecting the specimen to extension unloading, i.e., increasing lateral stress while holding vertical stress constant, or to extension loading, i.e., decreasing vertical stress while holding lateral stress constant. Other bulk solids such as agricultural grains, coal, cement powder, ores and polyethyene pellets will exhibit a similar sensitivity of stress-strain behavior to the method of loading.
The finite element method of determining stress-strain relationships is important in the design of structures in contact with soil masses. This method requires complex constitutive equations on stress strain characteristics of the soil. Similarly, storage facilities for bulk solids may be designed by finite element if the stress-strain behavior or the material to be stored is known.
One problem in using conventional triaxial equipment for determining the stress histories of particulate materials is that the equipment is designed to evaluate soil material at stress ranges much higher than the stresses which exist in other bulk solid storage facilities. A second problem is that the confining stress used in such equipment is often applied via an incompressible liquid which imparts a hydrostatic variation in stress along the length of the specimen. Although this variation may be negligible in soil mechanics applications, it can introduce serious errors in the case of other bulk solids. A final problem is that the individual particle sizes of other particulate materials, in particular agricultural grains, are much larger than those of soil. The size differences require a test cell which allows for accurate measurements of low stress levels while using a large specimen to minimize the effects of individual particle behavior.
When conventional triaxial testing equipment is used in conducting zero lateral strain tests, a measuring device, such as a caliper, is placed around the circumference of the test specimen. Alternatively, complicated equations are used to calculate the volume change in the triaxial cell. The lateral strain measuring device has the limitation that it measures strain only at the diameter where the device is placed on the specimen and as such, ignores strains above and below the device. Also, the lateral strain measuring device applies small confining stresses to the specimen which produce misleading results when deformable particulate materials are being tested.
Therefore, it is a primary objective of the present invention to provide an apparatus for determining the stress-strain relationships of particulate material.
It is a further objective of the present invention to provide an apparatus capable of yielding accurate results when performing zero lateral strain tests constant volume tests, and constant lateral stress tests on particulate material.
It is a further objective of the present invention to provide an apparatus for performing zero lateral strain tests on particulate material without a circumferential strain measuring device or complicated and lengthy calculations.
It is a further objective of the present invention to provide an apparatus for conducting stress-strain tests on particulate material in which the confining stresses are applied by use of compressed air.
It is a further objective of the present invention to provide an apparatus for testing the stress-strain relationships of particulate material that is easy to use and provides accurate results.